1. Field of the Invention
The invention relates to an engine cooling apparatus that effectively warms and cools the engine.
2. Description of the Related Art
When warming-up has not been completed, for example, great friction is caused in an engine. Therefore, the warming-up needs to be quickly completed. In particular, components having slide portions, such as cylinder bores, pistons, and a crankshaft, need to be quickly warmed to efficiently operate the engine. In an ordinary engine, when air-fuel mixture starts to be ignited in the cylinder, engine components such as a cylinder block and a cylinder head, and engine oil that is circulated in oil passages formed in the engine components, and coolant that is circulated in a water jacket are warmed. Thus, the warming-up gradually proceeds.
Also, a cooling apparatus is provided in an engine to avoid an excessive increase in the temperature of each portion of the engine after the warming-up is completed.
There are various types of engine cooling apparatuses. For example, a so-called separation cooling system, which separately cools a cylinder block and a cylinder head, is proposed. Further, Japanese Patent Application Publication No. 2004-346928 (JP-A-2004-346928) describes a cooling system that is configured by improving the separation cooling system.
To reduce the amount of pollutants in exhaust gas, it is necessary to improve the effect of cooling the cylinder head of an engine. Also, it is necessary to improve the effect of warming the cylinder block during cold start. In the above-described separation cooling system, the effect of cooling the cylinder head is improved. However, because a water pump is generally provided in the cylinder block of an engine, a pipe or a jacket needs to be newly provided so that the coolant flows directly into a head water jacket from the water pump. In this case, the heat capacity of the pipe or the jacket that is newly provided is added to the heat capacity of the block water jacket. Therefore, the heat capacity is increased. This increase in the heat capacity is not desirable, because the effect of warming the cylinder block needs to be improved. Such a problem may arise also in the engine cooling system described in the above-described publication. Thus, improvement needs to be made to achieve both of the effect of cooling the cylinder head and the effect of warming the cylinder block during engine cold start.